Dodgers and baseball question

Looks like we're headed to the WS. Congratulations to Roberts.

Question: How long does one have to hold onto a ball before its ruled a catch? I'm referring to the drive Bellinger hit late in the game that was ruled a drop. Seemed to me the outfielder caught it and then when turning to throw it it dropped from his glove. Why wasn't that an out and then a possible error on the OF ?